The present invention generally relates to burners, and more particularly relates to burners for use in air heating applications.
Burners for heating air typically comprise a fuel manifold having a plurality of linearly aligned fuel discharge ports. A pair of mixing plates or wings are attached to the manifold and have combustion air ports extending therethrough to produce jets of air which mix with fuel exiting the discharge ports to create a combustible mixture. In most air heating burners, the mixing plates generally diverge from one another in the downstream direction to form a V-shape.
It is important for such burners to have a wide turndown range to provide varying heat outputs to the process. It will be appreciated that as fuel flow is adjusted along the turndown range, the velocity of fuel flow through the discharge ports changes in a corresponding fashion. At low fire, for example, the fuel has a relatively low velocity. As such, air from only the upstream ports of the plates mixes with the fuel to produce a combustible mixture. As the firing rate increases, the fuel velocity similarly increases to project fuel further downstream between the plates. As a result, air from further downstream ports mixes with the fuel to increase the amount of combustible mixture. Mixture of fuel and air begins immediately at each combustion air port to produce localized combustion adjacent the air port.
It is particularly important for air heaters to minimize the level of CO emissions generated during operation. When air is supplied at a constant volume, as is conventional and most cost effective (because no combustion air flow controls are required), the above-described conventional burners fail to meet acceptable emission levels over the entire range of inputs. At high fire, conventional air heaters are designed to have approximately ten to fifty percent (10-50%) excess air, resulting in substantially complete combustion and acceptable CO emissions of less than about 400 ppm. Due to the fixed volume of air, however, the proportion of excess air becomes extremely high for low to medium-low inputs. Combustion at these levels is often quenched by the high amount of excess air, leading to a lower combustion temperature, incomplete combustion, and unacceptably high CO levels of 1000 ppm or more.
In the past, others have attempted to address this problem by optimizing combustion air port size and location. Such optimization may lower CO emissions at medium to high fire, but CO levels at lower inputs are still unsatisfactory. The CO emission profile across a range of inputs follows the same pattern, in which the highest CO emissions are seen at lower inputs. Furthermore, conventional burners produce widely varying CO emissions according to the magnitude of the pressure drop across the burner. In general, previous burners tend to generate lower CO emissions with smaller pressure drops, while greater pressure drops are often associated with higher CO levels.
A general object of the present invention is to provide an air heating burner which produces relatively low CO emissions over an entire range of inputs.
A related object of the present invention is to provide an air heating burner which minimizes CO emissions during low to medium-low fire.
In that regard, it is an object of the present invention to provide an air heating burner that eliminates quenching of combustion by excess air during low to medium-low fire conditions.
It is also an object of the present invention to provide an air heating burner having consistently low CO emissions regardless of the magnitude of the pressure drop across the burner and thereby air flow velocity through the ports of the mixing plates.
An additional object of the present invention is to provide an air heating burner having more uniform combustion air flow distribution.
Yet another object of the present invention is to provide an air heating burner which pre-heats combustion air to increase overall combustion temperature, thereby reducing CO emissions.
Still another object of the present invention is to provide an air heating burner which reduces flame length at high fire.
In light of the above, an air heating burner is provided which reduces combustion air velocity in the region of the burner associated with low to medium-low fire. The burner assembly includes a fuel manifold having mixing plates attached thereto and disposed downstream of the manifold. The mixing plates diverge from one another in a downstream direction to form a general V-shape when viewed from above. A plurality of combustion air ports extends through the mixing plates to provide jets of air for mixing with fuel from the manifold. In the currently preferred embodiment, the burner assembly includes a pair of wings located upstream of and overlapping portions of associated mixing plates. The wings cover the upstream (or lower) portion of the mixing plates, which carry the combustion air ports associated with low to medium-low fire. Air flow ports in the wings provide an additional pressure drop which decreases air flow velocity at the mixing plates. The wings do not cover the combustion air ports associated with medium to high fire. The lower air flow velocity through the low and medium-low air ports reduces quenching, thereby minimizing CO emissions. The wings also ensure that fuel flow velocity is greater than air flow velocity over a larger operating range, thereby maximizing the turndown of the burner. Each wing with its associated mixing plate also forms a pre-heat chamber for the incoming combustion air. As a result, the temperature of combustion is increased, which also serves to minimize CO emissions.
The burner is also configured to reduce flame length at high fire. The extreme downstream portion of each mixing plate extends directly downstream so that the mixing plates are parallel to one another in this area. These areas of the mixing plates carry combustion air ports associated with high fire. The parallel mixing plate portions serve to reduce flame length during high fire, thereby reducing space requirements for the burner application. The parallel portions also create more intense mixing of air with fuel and combustion products, thereby creating more complete combustion and reducing CO emissions.
These and other aims, objectives, and features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.